a blog about building a plane…

Monthly Archives: August 2017

Although I had run airspeed calibration tests earlier in my flight testing, I was never satisfied with the accuracy because of the turbulence I always seemed to encounter. So, I decided to re-run these tests early yesterday morning, before the sun had time to heat things up.

I was at the airport at sun up and launched shortly after that. The technique I used is the well-known three-leg GPS process by Doug Gray. This technique is documented in multiple places on the web, but one good place to access this is at Kevin Horton’s site here. Essentially, you fly three legs with different headings (preferentially 90 to 120 degrees from each other), note the GPS ground speed and GPS track. You plug these numbers into the calculations shown in Doug’s paper and voila, it spits out your true airspeed, While flying these tracks, you should note your true airspeed as shown on your avionics.

I ran tests at 4,000 and 8,000 feet. Here are the numbers as noted in the air:

4000 ft

Leg 1

Leg 2

Leg 3

GPS GS

162

146

118

GPS Track

150

030

290

IAS

134

131

132

TAS

146

142

143

Heading

157

020

287

PA

3960

3960

3960

OAT(C)

24

23

24

After plugging these numbers into the calculation, I was surprised to see that the true airspeed was calculated as 144 kts. The average of the above three TAS numbers is 143.67, so that’s not too bad.

Here are the 8000 ft results:

8000 ft

Leg 1

Leg 2

Leg 3

GPS GS

149

177

160

GPS Track

180

090

010

IAS

133

132

134

TAS

153

152

155

Heading

192

094

359

PA

7960

7960

7960

OAT(C)

15.6

15

15.6

The calculated result was 154 kts vs. an average TAS noted of 153.33. I’ll take it!

I was astounded that these numbers came out so close, especially considering the ham-handed way I installed the static ports, but I guess a blind hog finds an acorn every once in a while.

The plane is too slow, but the wheel pants haven’t been installed yet and there are other clean up tasks to perform so I’m not worried about it at this point.